This invention relates generally to switching amplifiers and more particularly to preventing simultaneous conduction of the individual amplifier elements in a push-pull switching amplifier.
The use of push-pull switching amplifiers in a square wave or pulse type of operation has proven to be an effective means of achieving high efficiencies in amplifiers as well as inverters. In their basic form of operation, the two individual amplifier elements, which may be vacuum tubes or semiconductors, are alternately switched at a fundamental frequency to conduct an input signal for 180.degree. intervals. A modification of this which exhibits high efficiencies is the use of Class S amplifiers which use some form of pulse width modulation (PWM), pulse duration modulation (PDM) or time-ratio controlled modulation to superimpose a higher switching frequency upon the fundamental frequency. The higher switching frequency causes each one of the amplifier elements to switch on and off several times during each of its half cycles of the fundamental frequency. Principles of Inverter Circuits by B. D. Bedford and R.G. Hoft, published by John Wiley and Sons, 1964 gives a good illustration of typical equations and waveforms R. G. Class S amplifiers on pages 256-262.
One problem which exists in push-pull switching amplifiers is simultaneous conduction by the amplifier elements rather than the desired purely alternate operation. Simultaneous conduction occurs during switching because stray, lumped and distributed capacitances in the amplifier discharge through the amplifier element if it is driven even though that particular amplifer element cannot contribute useful power during that portion of the fundamental frequency period. This conduction represents a power loss to the amplifier and reduces its efficiency. The problem tends to be worse in Class S amplifiers because there are more switching intervals.
Previously, proposals have been made to eliminate simultaneous conduction in push-pull amplifiers. One such proposal has been the use of a drive circuit to enable one amplifier element while inhibiting the other. The drive is based on a feedback signal from the amplifier output which indicates which amplifier element is on at the moment. As long as it remains on, the other amplifier element will be inhibited. When the conductive element completely shuts off, the other element is allowed to conduct and the original element is inhibited. U.S. Pat. Nos. 3,448,395 and 3,825,773 use this type of drive circuit. These devices, while useful, have failed to provide any control of the drive based on the phase of the amplifier output signals. Such an omission can lead to inefficiencies in operation if there has been a phase shift between the amplifier input and output. Such phase shifts are common across the amplifier elements, especially in the plate circuits of high-power vacuum tubes. Further phase shifting can occur across the output transformer if one is used and/or across output filter components if they are used.